This invention relates generally to gas turbine engines and more particularly, to a method and apparatus used to regulate fluid flow for a gas turbine engine.
At least some known gas turbine engines include an engine casing that extends circumferentially around a compressor, and a turbine including a rotor assembly and a stator assembly. Within some engines, a plurality of ducting and valves coupled to an exterior surface of the casing are used to channel fluid flow from one area of the engine for use within another area of the engine. For example, such ducting and valves may form a portion of an environmental control system (ECS).
Typically, valve assemblies are used to control fluid flow that is at a high temperature and/or high pressure. Such valve assemblies include a substantially cylindrical valve body that is coupled between adjacent sections of ducting. The valve body includes a valve sealing mechanism that is selectively positionable to control fluid flow through the valve. More specifically, at least some known valves include a single piston/cylinder arrangement that is positioned external to the valve body and is coupled to the valve sealing mechanism to provide the motive force necessary to selectively position the valve sealing mechanism.
Because the piston/cylinder arrangement is offset from the main valve body, a center of gravity of the valve assembly is typically displaced a distance from a centerline axis of the valve body. Such an eccentric center of gravity may induce bending stresses into the valve assembly, adjoining tubing, and supporting brackets during engine operation. Depending on the application, the physical size and weight of the piston/cylinder arrangement may also present difficulties during the duct routing phase of the engine design.
A concentric valve is one way to address such difficulties. The concentric valve as described in U.S. Pat. Nos. 6,775,990 and 6,986,257 features an actuation piston that surrounds the flowbody of the valve, hence the name. This piston is sized so that the inner and outer diameters of the piston form an actuation area that fluid pressure works against. This area is typically set to achieve a force margin of at least 3:1, after accounting for all resistive forces. The fluid pressure level acting on the piston is usually set by the system architecture. In some cases, the actuation fluid (fuel) pressure is reasonably low, at about 130 psid for example. For other cases, the fluid pressure is much higher, at up to about 900 psid.
The pressure level and other geometrical constraints can create a situation where it is not practical to have an actual concentric piston surrounding the flowbody. Cases of extremely high pressure may necessitate a piston with a wall thickness of only 0.030″ to achieve a 3:1 force margin. Such a constraint may not be practical as there is not enough space for guide seal glands, and it is not desirable to place the glands in the housing walls. Additionally, to handle the high burst pressure conditions (3000 psia in some cases) the actuator housing walls have to be sized to resist the hoop stresses. While it is possible to make a concentric valve for such applications there is a penalty incurred in the form of weight and packaging volume.